May 2011
Intermediate to advanced
788 pages
23h 34m
English
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GRE and Access Lists
GRE is a protocol on the same level as TCP and UDP. When configuring
a firewall to allow GRE, you do not configure a port like you would for
Telnet or SSH. Instead, you must configure the firewall to allow protocol
47. Cisco routers offer the keyword gre for configuring access lists:
R1(config)#access-list 101 permit ?
<0-255> An IP protocol number
ahp Authentication Header Protocol
eigrp Cisco's EIGRP routing protocol
esp Encapsulation Security Payload
gre Cisco's GRE tunneling
icmp Internet Control Message Protocol
igmp Internet Gateway Message Protocol
igrp Cisco's IGRP routing protocol
ip Any Internet Protocol
ipinip IP in IP tunneling
nos KA9Q NOS compatible IP over IP tunneling
ospf OSPF routing protocol
pcp Payload Compression Protocol
pim Protocol Independent Multicast
tcp Transmission Control Protocol
udp User Datagram ProtocolPIX firewalls also support the keyword gre:
PIX(config)#access-list In permit gre host 10.10.10.10 host 20.20.20.20The Point-to-Point Tunneling Protocol (PPTP) uses GRE, so if you’re using this protocol for VPN access, you will need to allow GRE on your firewall.
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