BGP link-bw & multipath Load Balancing

An autonomous system can be connected to another through multiple links and according to the company business and redundancy requirements different schemes can be used:

- Primary/secondary: where the second link is used only when the first link fails.

- Symmetric load-sharing: where the traffic is equally distributed among multiple links in the same time, which provides a high level of redundancy for the enterprise.

But, it’s not always possible to provide equal bandwidth links because of either financial limits or availability of such solution. So the need to engineer traffic through these links according to their bandwidth capacity.

Here comes the solution of BGP link bandwidth.

With the deployment of BGP multipath, generally the decision of using multiple path to deliver the traffic is performed inside the autonomous system by an iBGP according to multiple criteria excluding the eBGP link bandwidth.

BGP link-bw advertise bandwidth of an autonomous system exit link as extended community to iBGP.

Some requirements are to be considered:

- Only between directly connected eBGP peers.

- BGP extended community should be enabled between iBGP.

- CEF should be enabled everywhere.

Figure 1 illustrates the lab topology used to implement BGP link-bw

Figure1: Topology

Inside AS 64540, R1, R2 and R3 establish full mesh iBGP sessions, the same for AS 64550: R4, R5, R6 and R7 establish full mesh iBGP sessions.

Links R2-R4, R5-R3, R6-R3 are direct eBGP sessions using interfaces ip addresses as sources and destinations.

Network default behavior

The network default configuration is as follow:

AS 64540:

R1:

R1(config-router)#do sh ip bgp BGP table version is 3, local router ID is 10.10.10.1 Status codes: s suppressed, d damped, h history, * valid, > best, i – internal, r RIB-failure, S Stale Origin codes: i – IGP, e – EGP, ? – incomplete Network Next Hop Metric LocPrf Weight Path *> 10.10.10.0/24 0.0.0.0 0 32768 i * i70.70.70.0/24 3.3.3.3 0 100 0 64550 i *>i 2.2.2.2 0 100 0 64550 i R1(config-router)#

R1(config-router)#do sh ip bgp 70.70.70.0 BGP routing table entry for 70.70.70.0/24, version 3 Paths: (2 available, best #2, table Default-IP-Routing-Table) Not advertised to any peer 64550 3.3.3.3 (metric 2297856) from 3.3.3.3 (3.3.3.3) Origin IGP, metric 0, localpref 100, valid, internal 64550 2.2.2.2 (metric 2297856) from 2.2.2.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, internal, best R1(config-router)#

the default path chosen is through R2-R4:

R1(config-router)#do traceroute 70.70.70.1 source 10.10.10.1 Type escape sequence to abort. Tracing the route to 70.70.70.1 1 192.168.12.2 24 msec 320 msec 452 msec 2 192.168.24.2 1004 msec 716 msec 484 msec 3 192.168.47.2 292 msec * 556 msec R1(config-router)#

So the traffic from R1 to R7 takes the path R1-R2-R7

Table1: best path selection for 70.70.70.1/24 from R1

	Attribute	Path1	Path2
1	weight	0	0
2	local preference	100	100
3	originated locally	No	No
4	AS_PATH	64550	64550
5	ORIGIN	i	i
6	MED	0	0
7	eBGP<>iBGP	iBGP	iBGP
8	Best IGP metric to NEXT-HOP	2297856	2297856
9	Multipath	No	No
10	oldest path	No	No
11	Lowest neighbor router-ID	3.3.3.3	2.2.2.2 <<<

AS 64550:

R7:

R7(config-router)#do sh ip bgp BGP table version is 3, local router ID is 70.70.70.1 Status codes: s suppressed, d damped, h history, * valid, > best, i – internal, r RIB-failure, S Stale Origin codes: i – IGP, e – EGP, ? – incomplete Network Next Hop Metric LocPrf Weight Path * i10.10.10.0/24 5.5.5.5 0 100 0 64540 i *>i 4.4.4.4 0 100 0 64540 i * i 6.6.6.6 0 100 0 64540 i *> 70.70.70.0/24 0.0.0.0 0 32768 i R7(config-router)#

R7(config-router)#do traceroute 10.10.10.1 source 70.70.70.1 Type escape sequence to abort. Tracing the route to 10.10.10.1 1 192.168.47.1 8 msec 268 msec 104 msec 2 192.168.24.1 164 msec 348 msec 136 msec 3 192.168.12.1 276 msec * 260 msec R7(config-router)#

So the traffic from R7 to R1 takes the path R7-R4-R2-R1

R7(config-router)#do sh ip bgp 10.10.10.0 BGP routing table entry for 10.10.10.0/24, version 3 Paths: (3 available, best #2, table Default-IP-Routing-Table) Not advertised to any peer 64540 5.5.5.5 (metric 2297856) from 5.5.5.5 (5.5.5.5) Origin IGP, metric 0, localpref 100, valid, internal 64540 4.4.4.4 (metric 2297856) from 4.4.4.4 (4.4.4.4) Origin IGP, metric 0, localpref 100, valid, internal, best 64540 6.6.6.6 (metric 2297856) from 6.6.6.6 (6.6.6.6) Origin IGP, metric 0, localpref 100, valid, internal R7(config-router)#

R4-R2 link is chosen as the best path to reach the prefix 10.10.10.1/24:

Table2: best path selection for 10.10.10.1/24 from R7

	Attribute	Path1	Path2	Path3
1	weight	0	0	0
2	local preference	100	100	100
3	originated locally	No	No	No
4	AS_PATH	64540	64540	64540
5	ORIGIN	i	i	i
6	MED	0	0	0
7	eBGP<>iBGP	iBGP	iBGP	iBGP
8	Best IGP metric to NEXT-HOP	2297856	2297856	2297856
9	Multipath	No	No	No
10	oldest path	No	No	No
11	Lowest neighbor router-ID	5.5.5.5	4.4.4.4 <<<	6.6.6.6

BGP Link-BW deployment

The best way to utilize BW resources is to load-share the traffic among the three eBGP link according to their BW:

let’s recall the requirements for using BGP link BW:

- Requires BGP multipath configured.

- Enable BGP ext. community between iBGP.

- Enable CEF everywhere.

General configuration:

On each iBGP speaker with multilink ramification, enable iBGP multipath

router bgp maximum-paths maximum-paths ibgp

router bgp address-family ipv4 neighbor activate neighbor send-community extended !iBGP peer to which extended community is to be send. neighbor activate neighbor dmzlink-bw !Allow eBGP bandwidth to be propagated through link-bw extended community bgp dmzlink-bw !“bgp dmzlink-bw” is configured on any router whose eBGP link bandwidth !will be used for load-balancing. exit-address-family

As 65540:

R1(iBGP):

router bgp 64540 address-family ipv4 neighbor 2.2.2.2 activate neighbor 3.3.3.3 activate maximum-paths 3 maximum-paths ibgp 3 exit-address-family

eBGP speaker R2:

router bgp 64540 address-family ipv4 neighbor 1.1.1.1 activate neighbor 1.1.1.1 send-community extended neighbor 1.1.1.1 next-hop-self neighbor 3.3.3.3 activate neighbor 3.3.3.3 next-hop-self neighbor 192.168.24.2 activate neighbor 192.168.24.2 dmzlink-bw bgp dmzlink-bw exit-address-family

eBGP speaker R3:

router bgp 64540 address-family ipv4 neighbor 1.1.1.1 activate neighbor 1.1.1.1 send-community extended neighbor 1.1.1.1 next-hop-self neighbor 2.2.2.2 activate neighbor 2.2.2.2 next-hop-self neighbor 192.168.35.2 activate neighbor 192.168.35.2 dmzlink-bw neighbor 192.168.36.2 activate neighbor 192.168.36.2 dmzlink-bw maximum-paths 2 maximum-paths ibgp 2 bgp dmzlink-bw exit-address-family

Verification:

R1#sh ip route Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2 E1 – OSPF external type 1, E2 – OSPF external type 2 i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2 ia – IS-IS inter area, * – candidate default, U – per-user static route o – ODR, P – periodic downloaded static route Gateway of last resort is not set 192.168.12.0/30 is subnetted, 1 subnets C 192.168.12.0 is directly connected, Serial1/0 1.0.0.0/32 is subnetted, 1 subnets C 1.1.1.1 is directly connected, Loopback0 192.168.13.0/30 is subnetted, 1 subnets C 192.168.13.0 is directly connected, Serial1/1 2.0.0.0/32 is subnetted, 1 subnets D 2.2.2.2 [90/2297856] via 192.168.12.2, 03:20:35, Serial1/0 70.0.0.0/24 is subnetted, 1 subnets B 70.70.70.0 [200/0] via 3.3.3.3, 01:11:12 [200/0] via 2.2.2.2, 01:11:12 3.0.0.0/32 is subnetted, 1 subnets D 3.3.3.3 [90/2297856] via 192.168.13.2, 03:20:29, Serial1/1 10.0.0.0/24 is subnetted, 1 subnets C 10.10.10.0 is directly connected, Loopback1 R1#

R1#sh ip route 70.70.70.1 Routing entry for 70.70.70.0/24 Known via “bgp 64540″, distance 200, metric 0 Tag 64550, type internal Last update from 2.2.2.2 01:08:48 ago Routing Descriptor Blocks: 3.3.3.3, from 3.3.3.3, 01:08:48 ago Route metric is 0, traffic share count is 1 AS Hops 1 Route tag 64550 * 2.2.2.2, from 2.2.2.2, 01:08:48 ago Route metric is 0, traffic share count is 1 AS Hops 1 Route tag 64550 R1#

R1:

R1#sh ip bgp 70.70.70.1 BGP routing table entry for 70.70.70.0/24, version 7 Paths: (2 available, best #2, table Default-IP-Routing-Table) Multipath: eBGP iBGP Not advertised to any peer 64550 3.3.3.3 (metric 2297856) from 3.3.3.3 (3.3.3.3) Origin IGP, metric 0, localpref 100, valid, internal, multipath DMZ-Link Bw 1443 kbytes 64550 2.2.2.2 (metric 2297856) from 2.2.2.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, internal, multipath, best DMZ-Link Bw 12500 kbytes R1#

Note the proportion of the link BW of path 2 (through 2.2.2.2) against link BW of path 1 (through 3.3.3.3).

Table3: best path selection for 70.70.70.1/24 from R1 after BGP Link-bw

	Attribute	Path1	Path2
1	weight	0	0
2	local preference	100	100
3	originated locally	No	No
4	AS_PATH	64550	64550
5	ORIGIN	i	i
6	MED	0	0
7	eBGP<>iBGP	iBGP	iBGP
8	Best IGP metric to NEXT-HOP	2297856	2297856
9	Multipath	2 <<<<	2 <<<<
10	oldest path	No	No
11	Lowest neighbor router-ID	3.3.3.3	2.2.2.2

R3:

R3#sh ip route Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2 E1 – OSPF external type 1, E2 – OSPF external type 2 i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2 ia – IS-IS inter area, * – candidate default, U – per-user static route o – ODR, P – periodic downloaded static route Gateway of last resort is not set 192.168.12.0/30 is subnetted, 1 subnets D 192.168.12.0 [90/2681856] via 192.168.13.1, 03:21:04, Serial1/0 1.0.0.0/32 is subnetted, 1 subnets D 1.1.1.1 [90/2297856] via 192.168.13.1, 03:21:04, Serial1/0 192.168.13.0/30 is subnetted, 1 subnets C 192.168.13.0 is directly connected, Serial1/0 2.0.0.0/32 is subnetted, 1 subnets D 2.2.2.2 [90/2809856] via 192.168.13.1, 03:21:04, Serial1/0 70.0.0.0/24 is subnetted, 1 subnets B 70.70.70.0 [20/0] via 192.168.35.2, 01:11:47 [20/0] via 192.168.36.2, 01:11:47 3.0.0.0/32 is subnetted, 1 subnets C 3.3.3.3 is directly connected, Loopback0 10.0.0.0/24 is subnetted, 1 subnets B 10.10.10.0 [200/0] via 1.1.1.1, 01:18:16 192.168.36.0/30 is subnetted, 1 subnets C 192.168.36.0 is directly connected, Serial1/1 192.168.35.0/30 is subnetted, 1 subnets C 192.168.35.0 is directly connected, Ethernet0/0 R3#

R3#sh ip route 70.70.70.1 Routing entry for 70.70.70.0/24 Known via “bgp 64540″, distance 20, metric 0 Tag 64550, type external Last update from 192.168.36.2 01:09:28 ago Routing Descriptor Blocks: * 192.168.35.2, from 192.168.35.2, 01:09:28 ago Route metric is 0, traffic share count is 1 AS Hops 1 Route tag 64550 192.168.36.2, from 192.168.36.2, 01:09:28 ago Route metric is 0, traffic share count is 1 AS Hops 1 Route tag 64550 R3#

R3#sh ip bgp 70.70.70.1 BGP routing table entry for 70.70.70.0/24, version 6 Paths: (3 available, best #1, table Default-IP-Routing-Table) Multipath: eBGP iBGP Advertised to update-groups: 1 2 3 64550 192.168.35.2 from 192.168.35.2 (5.5.5.5) Origin IGP, localpref 100, valid, external, multipath, best DMZ-Link Bw 1250 kbytes 64550 2.2.2.2 (metric 2809856) from 2.2.2.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, internal 64550 192.168.36.2 from 192.168.36.2 (6.6.6.6) Origin IGP, localpref 100, valid, external, multipath DMZ-Link Bw 193 kbytes R3#

Note the proportion of the link BW of path 1 (through 192.168.35.2) against link BW of path 1 (through 192.168.36.2).

AS 64550:

The same configuration can be done for AS 64550 to have a symmetric traffic flow between the two ASs:

R4:

R4#bgpcf router bgp 64550 address-family ipv4 neighbor 5.5.5.5 activate neighbor 6.6.6.6 activate neighbor 7.7.7.7 activate neighbor 7.7.7.7 send-community extended neighbor 192.168.24.1 activate neighbor 192.168.24.1 dmzlink-bw bgp dmzlink-bw exit-address-family

R5:

bgp 64550 address-family ipv4 neighbor 4.4.4.4 activate neighbor 6.6.6.6 activate neighbor 7.7.7.7 activate neighbor 7.7.7.7 send-community extended neighbor 192.168.35.1 activate neighbor 192.168.35.1 dmzlink-bw bgp dmzlink-bw exit-address-family

R6:

router bgp 64550 address-family ipv4 neighbor 4.4.4.4 activate neighbor 5.5.5.5 activate neighbor 7.7.7.7 activate neighbor 7.7.7.7 send-community extended neighbor 192.168.36.1 activate neighbor 192.168.36.1 dmzlink-bw bgp dmzlink-bw exit-address-family

R7:

router bgp 64550 address-family ipv4 neighbor 4.4.4.4 activate neighbor 5.5.5.5 activate neighbor 6.6.6.6 activate maximum-paths 3 maximum-paths ibgp 3 exit-address-family

R7#sh ip bgp 10.10.10.1 BGP routing table entry for 10.10.10.0/24, version 9 Paths: (3 available, best #3, table Default-IP-Routing-Table) Multipath: eBGP iBGP Flag: 0×800 Not advertised to any peer 64540 5.5.5.5 (metric 2297856) from 5.5.5.5 (5.5.5.5) Origin IGP, metric 0, localpref 100, valid, internal, multipath DMZ-Link Bw 1250 kbytes 64540 6.6.6.6 (metric 2297856) from 6.6.6.6 (6.6.6.6) Origin IGP, metric 0, localpref 100, valid, internal, multipath DMZ-Link Bw 193 kbytes 64540 4.4.4.4 (metric 2297856) from 4.4.4.4 (4.4.4.4) Origin IGP, metric 0, localpref 100, valid, internal, multipath, best DMZ-Link Bw 12500 kbytes R7#

Table4: best path selection for 10.10.10.1/24 from R7 after configuring BGP link-bw

	Attribute	Path1	Path2	Path3
1	weight	0	0	0
2	local preference	100	100	100
3	originated locally	No	No	No
4	AS_PATH	64540	64540	64540
5	ORIGIN	i	i	i
6	MED	0	0	0
7	eBGP<>iBGP	iBGP	iBGP	iBGP
8	Best IGP metric to NEXT-HOP	2297856	2297856	2297856
9	Multipath	3 <<<<	3 <<<<	3 <<<<
10	oldest path	No	No	No
11	Lowest neighbor router-ID	5.5.5.5	4.4.4.4	6.6.6.6

CONCLUSION

BGP link-bw provides an optimal way to use link bandwidth resources between autonomous systems, make sure CEF is enabled (enabled by default), iBGP multipath is already configured and enable the propagation of the extended community to iBGP neighbors.