Cisco Cisco IOS Software Release 12.0(21)S
GRE Tunneling on Cisco 12000 Series Internet Routers
Configuration Tasks
9
Cisco IOS Release 12.0(21)S
Configuring the ToS Byte
You can optionally configure the ToS byte of the tunnel’s IP header. If you do not specify a ToS byte,
the three most significant bits (IP precedence bits) of the payload IP header are copied to the
corresponding bits of the tunnel’s carrier IP header.
the three most significant bits (IP precedence bits) of the payload IP header are copied to the
corresponding bits of the tunnel’s carrier IP header.
To configure the ToS byte used in IP packet headers:
Verifying GRE Tunnel Configuration
Step 1
Use the show running-config interface tunnel command to display the tunnel destination IP address
for interfaces.
for interfaces.
Router(config)# show running-config interface tunnel 4
Current configuration : 165 bytes
!
interface Tunnel4
ip address 7.7.7.7 255.255.255.0
no ip directed-broadcast
no ip route-cache cef
tunnel source Loopback1
tunnel destination 61.61.61.61
end
Step 2
Use the show ip route
command to verify the IP routes are valid. There should be a valid entry for the
tunnel destination address.
Router# show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
Gateway of last resort is not set
61.0.0.0/32 is subnetted, 1 subnets
S 61.61.61.61 [1/0] via 3.3.3.4
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
Gateway of last resort is not set
Command
Purpose
Router(config-if)# tunnel tos number
Sets the value of the ToS byte. Valid values: 0 - 255.
The three most significant bits of the ToS byte are called the
IP precedence bits. Most applications and vendors currently
support setting and recognizing these three bits.
IP precedence bits. Most applications and vendors currently
support setting and recognizing these three bits.
Example: To set an IP precedence value of 5 critical, you
need to set the highest three bits of the ToS byte with a binary
value of 10100000 (which is equivalent to the decimal value
of 160 needed in the command value).
need to set the highest three bits of the ToS byte with a binary
value of 10100000 (which is equivalent to the decimal value
of 160 needed in the command value).