BGP peering with 3750s

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Aug 14th, 2009
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We currently have two stacked 3750s which are connected to Cogent through a single BGP link.

We want to get our own AS number and peer with two providers.

Can the 3750 hold a full BGP routing table? does it need to? (must it be upgraded with more memory?) Is it even a good idea to use a 3750 class device in this way? Will the connection to the two providers be active/active or must it be active/passive?

Here is my switch info:

ca1-sw1#sh version

Cisco IOS Software, C3750E Software (C3750E-UNIVERSAL-M), Version 12.2(35)SE2, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2007 by Cisco Systems, Inc.

Compiled Mon 05-Feb-07 23:04 by antonino

Image text-base: 0x00003000, data-base: 0x01475054

ROM: Bootstrap program is C3750E boot loader

BOOTLDR: C3750E Boot Loader (C3750E-HBOOT-M) Version 12.2(35r)SE, RELEASE SOFTWARE (fc1)

ca1-sw1 uptime is 1 year, 44 weeks, 6 days, 21 hours, 45 minutes

System returned to ROM by power-on

System image file is "flash:c3750e-universal-mz.122-35.SE2/c3750e-universal-mz.122-35.SE2.bin"

License Level: ipservices Type: Permanent

Next reboot license Level: ipservices

cisco WS-C3750E-24TD (PowerPC405) processor (revision C0) with 245760K/16376K bytes of memory.

Processor board ID CAT1129WG7H

Last reset from power-on

7 Virtual Ethernet interfaces

1 FastEthernet interface

56 Gigabit Ethernet interfaces

4 Ten Gigabit Ethernet interfaces

The password-recovery mechanism is enabled.

512K bytes of flash-simulated non-volatile configuration memory.

Model revision number : C0

Motherboard revision number : B0

Model number : WS-C3750E-24TD-E

Version ID : V01

Hardware Board Revision Number : 0x01

Switch Ports Model SW Version SW Image

------ ----- ----- ---------- ----------

* 1 30 WS-C3750E-24TD 12.2(35)SE2 C3750E-UNIVERSAL-M

2 30 WS-C3750E-24TD 12.2(35)SE2 C3750E-UNIVERSAL-M

Switch 02


Switch Uptime : 1 year, 44 weeks, 6 days, 21 hours, 45 minutes

Model revision number : C0

Motherboard revision number : B0

Model number : WS-C3750E-24TD-E

Top assembly revision number : A0

Version ID : V01

License Level : ipservices

License Type : Permanent

Next reboot licensing Level : ipservices

Configuration register is 0xF


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Overall Rating: 4.8 (4 ratings)
Jerry Ye Fri, 08/14/2009 - 10:56
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Hi Ryan,

3750 cannot handle the full BGP table. Memory upgrade is not an option.

My question is why you want a full BGP table? Have you consider a partial table from the providers? With partial table, it can be active/active.



ryanbyrdcisco Fri, 08/14/2009 - 11:03
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Thank you, Jerry!

what are the downsides to a partial table?

any ideas on how to setup an active/active dual peering with a partial table?

this is what i did the first time to peer with cogent:

router bgp 64562

no synchronization

bgp log-neighbor-changes

network A.B.62.0 mask

network A.B.67.11 mask

network A.B.67.13 mask

neighbor A.B.61.177 remote-as 174

neighbor A.B.61.177 description Cogent's A Peer 1-3647471

neighbor A.B.61.177 password PASSWORD

neighbor A.B.61.177 version 4

neighbor A.B.62.121 remote-as 174

neighbor A.B.62.121 description Cogent's A Peer 1-3650376

neighbor A.B.62.121 password PASSWORD

neighbor A.B.62.121 version 4

neighbor A.B.67.10 remote-as 174

neighbor A.B.67.10 description Cogent's B Peer 1-3650376

neighbor A.B.67.10 password PASSWORD

neighbor A.B.67.10 ebgp-multihop 5

neighbor A.B.67.10 update-source Loopback10

neighbor A.B.67.12 remote-as 174

neighbor A.B.67.12 description Cogent's B Peer 1-3647471

neighbor A.B.67.12 password PASSWORD

neighbor A.B.67.12 ebgp-multihop 5

neighbor A.B.67.12 update-source Loopback20

maximum-paths 6

no auto-summary

where A.B was the first two octets and PASSWORD was the secret password

Jerry Ye Fri, 08/14/2009 - 11:15
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Hi Ryan,

With partial table, let's say two different ISP's are going to give you two different set of specific routes. When the traffic is hitting the router, it will pick the most specific path to the destination.

Of course, the ISP should also give you a default as the last resort.



Edison Ortiz Fri, 08/14/2009 - 11:20
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Let me clarify this passive/active active/active concept.

As long as you are peering with a BGP speaking router and the connection is up, you are in active state.

If you are peering with multiple BGP speaking devices, you will get routes from each BGP peer but if you get the same route from 2 different BGP peers, the BGP Best Path Selection kicks in:

And only one best route will be installed in the routing table.

Thus, the traffic leaving your network will only use one link while keeping the other BGP peer as backup.

You can install multiple best routes in the RIB with BGP by applying this command:




ryanbyrdcisco Fri, 08/14/2009 - 14:17
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does that mean that you can't do outbound traffic balancing with different upstream providers?

i'm told that equal-cost multipath doesn't work well with different upstreams. is that true?

Jerry Ye Fri, 08/14/2009 - 17:43
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You can still load balance the traffic with partial + default bgp tables, but it is not going to be 50/50. You need to manipulate your route policy carefully.

If you did what Edison mention, use full table and use maximum-paths eibgp, you can load balance the flow. Assuming the a prefix's AS-Path is equal. The problem this might introduce is large amount of out of sequence packets and retransmission, and depend on how the application can handle this type of traffic.



ryanbyrdcisco Fri, 08/14/2009 - 15:03
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Does this mean that if we peer with two providers (say, Cogent and XO), each 200mbps, that our outbound traffic would be limited to 200mbps because the other is used as a backup?

how might multipath (ECMP) or something similar be used to load balance the outgoing links so we could get 400mbps?

ryanbyrdcisco Fri, 08/14/2009 - 15:19
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Incidentatlly, are there any limitations to BGP peering if we get our own ASN versus using a private ASN?

Jerry Ye Fri, 08/14/2009 - 17:30
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The only caveat I can think of right now is make sure your own ASN is not configured as transit.



Edison Ortiz Fri, 08/14/2009 - 11:08
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The only reason you may want a full BGP routing table from 2 different providers is if you want to balance the traffic between the 2 links.

For instance, if you are trying to reach and provider A has a shorter AS PATH towards, traffic will be via provider A in both directions. If provider B has a shorter AS PATH towards other destinations, then provider B will be used.

You may lose this feature with partial routes or a default route, it's up to the provider.

If you decide to carry a partial route table, make sure to change the 'sdm template' to routing.

The default template only gives you 8,000 routes while the routing template gives you 11,000 routes.




ryanbyrdcisco Fri, 08/14/2009 - 14:19
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how do you control how large of a partial routing table the 3750 loads in?


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