Re: BGP 011: multiple sessions with upstreams

[Joe Abley <jabley () isc org>]

On 31 Dec 2004, at 11:01, Edward B. Dreger wrote:

> I'm trying to persuade them that two provider/customer BGP sessions is 
> a
> good thing,

The obvious reason for this might be explained along the lines of "your 
router can reach two of our routers. We'd like a BGP session to each so 
that we can take our routers down for maintenance when we feel like it, 
without causing routes to be withdrawn". If this doesn't gel with their 
policy, and you care about it, find a new ISP (but see below).

> and NEXT_HOP set to the HSRP-managed IP address would allow
> them to steer customer traffic.

I've made efforts to set this kind of thing up before (not at ISC), and 
in hindsight I wish I hadn't bothered. It was an unconventional 
configuration from the provider's perspective that was not preserved 
across provider router upgrades and configuration cleanups, and it 
opened us up to yet another failure mode (that of HSRP breakage, which 
is most definitely not unknown on many releases of IOS).

If you suffer unplanned outages so often that HSRP is going to make a 
noticeable difference, then it's probably a good idea to deal with 
whatever is causing the unplanned outages (unreliable telco, squirrels 
chewing through cables, poorly-sized UPSes, crimp-happy PBX monkeys).

If you're trying to protect against planned outages, then gracefully 
admin-downing the BGP sessions on the router which has impending 
maintenance (leaving BGP sessions to other router(s) up and running) 
might accomplish the same graceful failover that you're trying to use 
HSRP for.

> Their argument is that one can "get
> extra bandwidth" with two BGP sessions, and that HSRP prevents that.
> I've pointed out that equal-cost multipath BGP is far from default
> behavior, and that one can "get extra bandwidth" even without two BGP
> sessions.

An extra BGP session will not magically give you more bandwidth. An 
extra circuit will not-so-magically give you more bandwidth, although 
often not as much as you think.

> Am I missing something?

For your provider, supporting pur-laine, standard-configuration 
customers is cheaper than supporting customers where each has their own 
special-case setup. Supporting a network of routers where the protocols 
and configuration is consistent is also easier (and hence cheaper) than 
a network where each router has special, exciting new config bits found 
nowhere else.

Your choices may be:

1. Pay a premium to deal with an ISP who can really afford to support 
special-case customers;

2. Pay a cheaper price, and deal with reduced support and spontaneous 
mystery outages as engineers not familiar with your particular 
arrangement make general changes to their network;

3. Accept the standard setup, pay a cheaper price and get reasonable 
support.


Joe