Historical routing question
While giving a routing lecture today someone asked me "Why was eBGP assigned an administrative distance of 20 which is better than OSPF's administrative distance of 110. What was the logic behind that decision?" I was unable to think of an answer. Ideas? Thanks, Hank
Since AD is a vendor thing and not a standard, i guess you'll have to ask Cisco. My idea would be that since you are more likely to filter prefixes on ingress eBGP rather on IGP and eBGP usually carries the majority of prefixes, eBGP maybe was considered more "trustworthy" for the majority of traffic. In any case, eBGP AD can be changed (plus the network backdoor option) and some recommendations propose so. PS: If i remember correctly, Juniper does it the "right" way. -- Tassos Hank Nussbacher wrote on 10/4/2018 7:52 μμ:
While giving a routing lecture today someone asked me "Why was eBGP assigned an administrative distance of 20 which is better than OSPF's administrative distance of 110. What was the logic behind that decision?" I was unable to think of an answer. Ideas?
Thanks, Hank
Anno domini 2018 Hank Nussbacher scripsit: Hi,
While giving a routing lecture today someone asked me "Why was eBGP assigned an administrative distance of 20 which is better than OSPF's administrative distance of 110. What was the logic behind that decision?" I was unable to think of an answer. Ideas?
As eBGP usually is a connection to some elses network, and OSPF only is internally to your network, the idea is to get packets away to someone else (-> the destination) as fast/early as possible and avoid transporting the traffic in your own network if there is an exit. Best Max -- Friends are relatives you make for yourself.
Most likely there’s also some historic reason why it was implemented this way. BGP synchronisation is also one example where default was at early days to synchronise, and nowadays not to synchronise. Hot-potato routing can definitely be the reason for such a low AD for eBGP. And, as we nowadays run anycast services quite often we luckily have the BGP backdoor option to use to avoid the hot-potato if needed. Jome ----------------- Jorma Mellin Trustee SIY ry / ISOC Finland Chapter ENISA PSG member jorma@jmellin.net (tel. +358 50 9944762)
On 11 Apr 2018, at 12:36, Maximilian Wilhelm <max@rfc2324.org> wrote:
Anno domini 2018 Hank Nussbacher scripsit:
Hi,
While giving a routing lecture today someone asked me "Why was eBGP assigned an administrative distance of 20 which is better than OSPF's administrative distance of 110. What was the logic behind that decision?" I was unable to think of an answer. Ideas?
As eBGP usually is a connection to some elses network, and OSPF only is internally to your network, the idea is to get packets away to someone else (-> the destination) as fast/early as possible and avoid transporting the traffic in your own network if there is an exit.
Best Max -- Friends are relatives you make for yourself.
Hi, On Wed, Apr 11, 2018 at 12:47:16PM +0300, Jorma Mellin wrote:
Hot-potato routing can definitely be the reason for such a low AD for eBGP.
Since hot-potato only chooses "eBGP vs iBGP", AD has no relevance here. Unless you put your eBGP targets into your OSPF, and we all know that this is not a very good idea :-) Gert Doering -- NetMaster -- have you enabled IPv6 on something today...? SpaceNet AG Vorstand: Sebastian v. Bomhard, Michael Emmer Joseph-Dollinger-Bogen 14 Aufsichtsratsvors.: A. Grundner-Culemann D-80807 Muenchen HRB: 136055 (AG Muenchen) Tel: +49 (0)89/32356-444 USt-IdNr.: DE813185279
participants (5)
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Gert Doering -
Hank Nussbacher -
Jorma Mellin -
Maximilian Wilhelm -
Tassos Chatzithomaoglou